I. Field of the Invention
The present invention relates generally to respirometers. More particularly, the present invention relates to an on-line respirometer system and methods in which oxygen consumption by microorganisms, plants, or animals is measured by determining and monitoring the amount of injection oxygen required to maintain a constant concentration. Known prior art devices are found in United States Patent Class 366, Subclasses 101 and 102.
II. Description of the Prior Art
It has long been recognized by those skilled in the art that water contains a limited amount of oxygen. It is well known that respiring microorganisms, plants, or animals reduce oxygen concentrations within a given volume. Respirometers can monitor these biological reactions by measuring an exchange of gases, usually oxygen, carbon dioxide, or in the case of anaerobic reactions, methane. Such instruments have been used widely for assessing the response of microorganisms in wastewater treatment processes. The reduction in oxygen concentration is directly proportional to the amount of biodegradable organic matter available in wastewater.
A number of recent respirometers have been designed for on-line operation. These respirometers measure oxygen uptake or gas production in a continuous or semi-continuous mode. These measurements allow long-term changes in biological reactions to be monitored and recorded. Common applications of these prior art respirometers include monitoring oxygen uptake in wastewater treatment processes, monitoring the effect of changes in wastewater composition, and identifying the presence of toxic inputs that can adversely affect wastewater treatment processes.
Various respirometer designs are known in the art. Prior art respirometers typically comprise a reaction vessel, some method for introducing test wastewater, and a suitable device for monitoring the pressure change in the reaction vessel. The wastewater is introduced within the vessel along with microorganisms. The microorganisms grow and degrade the contaminants in the wastewater, thereby consuming oxygen and reducing oxygen concentration. Oxygen consumption is assumed to be the principal cause of pressure change within the reaction vessel. The reduction in oxygen is indicated by the pressure change in the reaction vessel over time.
There are four types of on-line respirometers available for commercial application. Type I on-line respirometers utilize a semi-continuous measuring method. These devices measure the change in dissolved oxygen concentration contained in a mixture of wastewater and microorganisms or the change in pressure within the respirometer. A long-term record of oxygen uptake is accomplished by conducting repetitive batch tests. In Type I respirometers an operator introduces a microorganism slurry along with a wastewater sample into the reaction vessel, adds oxygen, aerates for a short period of time, and finally measures the decrease in dissolved oxygen (DO) concentration or pressure drop. This difference between the initial and final dissolved oxygen concentrations or pressure drop corresponds to the amount of oxygen consumed by the microorganisms for the stabilization of the organic material present in the wastewater. The foregoing procedure is repeated several times and the results are plotted on a graph.
The latter procedure is represented by the following formula: EQU DO=DO.sub.0 -.intg.R.sub.s dt-.intg.R.sub.m dt, mg/l, where:
DO=Dissolved oxygen concentration in the sample at any time after beginning the test, mg/L PA1 DO.sub.0 =Dissolved oxygen concentration in the test sample at the beginning of the test, mg/L PA1 R.sub.s =Rate of oxygen uptake resulting from substrate (waste) oxidation, mg O.sub.2 /1-hr PA1 R.sub.m =Rate of oxygen uptake resulting from respiration of the microorganisms, mg O.sub.2 /1-hr PA1 OUR=R.sub.s +R.sub.m, mg O.sub.2 /L-hr PA1 SOUR=R.sub.s /M+R.sub.m /M, mg O.sub.2 /g. M-hr PA1 K.sub.La =Oxygen mass transfer coefficient PA1 DO.sub.s =Saturation value for dissolved oxygen under current test conditions, mg/L PA1 DO.sub.t =Dissolved oxygen concentration at any time, t, after addition of wastewater, mg/L PA1 .DELTA.DO=difference in dissolved oxygen concentration across reactor, mg/L PA1 V=Volume of reactor, L PA1 R.sub.w =Rate of wastewater inflow into the reaction vessel, L/hr PA1 K.sub.a =units coefficient PA1 .DELTA.%O=change in oxygen content of the headspace gas from gas inlet to outlet, % PA1 R.sub.a =Rate of air flow into the unit, L/hr PA1 V=Volume of culture slurry PA1 DO.sub.in =dissolved oxygen concentration in the influent wastewater microorganism slurry; PA1 DO.sub.out =dissolved oxygen concentration in culture discharge stream; PA1 k=units coefficient; PA1 O.sub.2in =percent oxygen in influent flushing gas stream; PA1 O.sub.2out =percent oxygen in effluent flushing gas stream; PA1 q.sub.w =liquid waste flow rate; PA1 q.sub.a =flow rate of flushing air into system; PA1 R.sub.o =the rate of oxygen input necessary to maintain a constant dissolved oxygen concentration or headspace oxygen content; PA1 R.sub.s =Rate of oxygen uptake due to substrate (waste) oxidation, mg O.sub.2 /1-hr., and, PA1 R.sub.m =Rate of oxygen uptake due to microorganism respiration, mg O.sub.2 /1-hr.
The resulting measurement may be represented as the oxygen uptake rate (OUR), expressed in terms of mass of oxygen per liter of mixture per hour (mg O.sub.2 /L-hr), or as the specific oxygen uptake rate (SOUR). In the SOUR test, the rate of oxygen consumption is expressed as the mass of oxygen per unit of microorganism mass, M, per hour (g. 0.sub.2 /g M/hr), or
Recording and plotting maximum OUR or SOUR versus time for successive batch tests creates a continuous presentation of responses to factors affecting microorganism growth. These factors include changes in waste concentration, composition, or toxicity as reflected by oxygen uptake rates. Knowledge of the background rate of respiration, Rm, allows calculation of the rate of oxidation, Rs, of the organic constituents of the wastewater sample. One problem with Type I respirometers is that the wastewater often must be diluted with aerated water before the measurement can take place. Another disadvantage is that the operation is limited to sequential batch reactions so that they do not provide a true record of continuous oxygen uptake. The oxygen uptake also can be measured by a manometric method, that is, by measuring the decrease in pressure within a sealed vessel containing the microorganisms and wastewater or by a decrease in the oxygen content of the head space gas.
In Type II on-line respirometers, the microorganism sample is mixed with air or oxygen enriched air within the reaction vessel during operation so that the dissolved oxygen concentration remains relatively constant. When a wastewater sample is added, the dissolved oxygen concentration temporarily begins to decrease in response to biodegradation in much the same manner as in Type 1 respirometers. However, in Type II respirometers the reduced dissolved oxygen concentration begins to increase as the rate of oxygen transferred into the culture medium exceeds the rate of oxygen uptake. The resulting dissolved oxygen curve is termed a "respirogram". A mass balance of dissolved oxygen concentration and mass transfer can be expressed in summary form as follows: EQU DO.sub.t =DO.sub.0 -.intg.R.sub.s dt-.intg.R.sub.m dt+.intg.K.sub.La (DO.sub.s -DO.sub.t)dt, mg/L,
where:
By measuring the dissolved oxygen concentration over time and using predetermined values for the microbial respiration and mass transfer reactions, the above equation may be used to calculate the oxygen uptake attributable to the biodegradation of wastewater constituents in the wastewater sample injected at the beginning of the test. The resulting mass of oxygen uptake is defined as the "short time biochemical oxygen demand", or BOD.sub.ST. A plot of successive measures of BOD.sub.ST indicates the change in wastewater quality over time. A plot of the oxygen uptake rate occurring immediately after the injection of wastewater sample, adjusted for microorganism respiration and mass transfer, gives a measure of maximum OUR. Several distinct disadvantages are associated with such respirometers. One disadvantage is the large number of repetitive measurements required to obtain a meaningful indication of wastewater quality. Another disadvantage is that the results are dependent on the mass transfer characteristics, which are subject to considerable variability over time.
In Type III on-line respirometers, oxygen uptake is measured as the decrease in dissolved oxygen concentration across a closed vessel that continuously receives a mixture of wastewater and microorganisms. Alternatively the microorganisms may be maintained within the vessel on a solid or semi-solid support medium. A decrease in dissolved oxygen concentration occurs almost immediately in response to changes in wastewater quality or composition. This approach provides a continuous measure of oxygen uptake rate that can be expressed as follows: EQU OUR=.DELTA.DO*R.sub.w /V, mg/L-hr,
where:
The data also may be processed to produce a measure of short term BOD (BOD.sub.ST). The BOD.sub.ST is obtained by correlating the oxygen uptake rate to standard measure of biochemical oxygen demand. Alternatively BOD.sub.ST may be calculated by comparison to the oxygen uptake of a synthetic substrate having a known BOD. One disadvantage of Type III respirometers is that the decrease in oxygen concentration across the reaction vessel must be relatively large to give an accurate measure of oxygen uptake. Another disadvantage of Type III systems that maintain a fixed medium is that the attached microorganisms may respond differently from those in the wastewater.
A Type IV on-line respirometer receives wastewater and microorganisms as in the Type III system but in an enclosed vessel having a defined headspace volume. Air is pumped into the headspace volume at a controlled rate and the resulting change in oxygen content of the gas phase is used as a measure of oxygen uptake rate as follows: EQU OUR=K.sub.a (.DELTA.%O)R.sub.a /V, mg/L-hr,
where
A specific disadvantage of Type IV respirometers is that the change in oxygen content of the air stream must be relatively large to provide an accurate measure of oxygen consumption by the microorganisms.
The known prior art respirometers are based on the principle of difference of dissolved oxygen in the culture slurry or headspace oxygen content or pressure existing within the reaction vessel over time due to consumption of oxygen by microorganisms in the contained sample. All known prior art respirometers require relatively large changes in dissolved oxygen concentration or headspace oxygen content to provide precise and accurate measurements of oxygen uptake of the test wastewater.
We have found it desirable to depart from the conventional techniques listed above. In our invention, oxygen uptake is determined directly by measuring the amount of oxygen that must be injected into a respirometer vessel to maintain an essentially constant dissolved oxygen concentration in the culture slurry or a near constant oxygen content in the headspace gas above the sample. The injection of oxygen is made in response to minute changes of oxygen in the containment vessel using oxygen concentration sensors within the vessel linked to suitable feedback computational devices.